Ethylene polymerization reaction steps

In Figure 2.5a insertion and elimination of ethylene in a M-H bond is shown. The steps from right to left are important in ethylene polymerization reactions. An example of the coexistence of isomers resulting from a- and -eliminations in the same complex is shown in Figure 2.5b. The spectator ligands of tantalum are not shown for clarity. 

Most chromium-based catalysts are activated in the beginning of a polymerization reaction through exposure to ethylene at high temperature. The activation step can be accelerated with carbon monoxide. Phillips catalysts operate at 85—110°C (38,40), and exhibit very high activity, from 3 to 10 kg HDPE per g of catalyst (300—1000 kg HDPE/g Cr). Molecular weights and MWDs of the resins are controlled primarily by two factors, the reaction temperature and the composition and preparation procedure of the catalyst (38,39). Phillips catalysts produce HDPE with a MJM ratio of about 6—12 and MFR values of 90—120. 

At 24 °C and 15-60 bar ethylene, [Rh(Me)(0H)(H20)Cn] catalyzed the slow polymerization of ethylene [4], Propylene, methyl acrylate and methyl methacrylate did not react. After 90 days under 60 bar CH2=CH2 (the pressure was held constant throughout) the product was low molecular weight polyethylene with Mw =5100 and a polydispersity index of 1.6. This is certainly not a practical catalyst for ethylene polymerization (TOP 1 in a day), nevertheless the formation and further reactions of the various intermediates can be followed conveniently which may provide ideas for further catalyst design. For example, during such investigations it was established, that only the monohydroxo-monoaqua complex was a catalyst for this reaction, both [Rh(Me)3Cn] and [Rh(Me)(H20)2Cn] were found completely ineffective. The lack of catalytic activity of [Rh(Me)3Cn] is understandable since there is no free coordination site for ethylene. Such a coordination site can be provided by water dissociation from [Rh(Me)(OH)(H20)Cn] and [Rh(Me)(H20)2Cn] and the rate of this exchange is probably the lowest step of the overall reaction.The hydroxy ligand facilitates the dissociation of H2O and this leads to a slow catalysis of ethene polymerization. 

TABLE 1. Ethylene Polymerization Scoping Reactions Using the Step 5 Catalyst with Hydrogen/Ethylene Molar Ratio of 0.2/0.8... 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

Bryant has calculated the changes in free energy for various reaction steps of the polymerization of tetrafluoroethylene. He concluded (1) that the initiation and propagation are about twice as favorable for tetrafluoroethylene as the analogous reactions for ethylene, (2) that termination by combination is more favorable than disproportionation, and (3) that chain-transfer to monomer and to polymer are less likely than the combination of radicals. 

Many kinetic studies on the high pressure ethylene polymerization are found in the literature (8-11). All authors agree on the following main reaction steps ... 

A large number of intermediate pathways arc possible when catalytic reactions interfere with the polymerization-dehydrogenation steps. A common scenario is the catalytic dehydrogenation of hydrocarbons on nickel surfaces followed by dissolution of the activated carbon atoms and exsolution of graphene layers after exceeding the solubility limit of carbon in nickel. Such processes have been observed experimentally [40] and used to explain the shapes of carbon filaments. In the most recent synthetic routes to nanotubes [41] the catalytic action of in situ-prepared iron metal particles was applied to create a catalyst for the dehydrogenation of cither ethylene or benzene. 

Historically, ethylene polymerization was carried out at high pressure (lUOO-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as bciizoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved halfarrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

Organic synthesis without solvents is already a mature field despite this, many chemists still assume that solvents are a necessity for most chemical processes. Therefore, the mindset of chemists needs to change and they must be willing to take up the opportunity that a solvent free method presents. Already, many multi-tonne industrial reactions are performed solvent free, particularly gas phase reactions such as ethylene polymerization. Although solid-solid reactions are yet to be performed on such a large scale, they have been performed on the kilogram scale. Also, solvent free approaches have recently been introduced into the multi-step synthesis of a potential antituberculosis drug,... 

The initial step in the mechanism of ethylene polymerization using Phillips catalysts is believed to occur by way of an oxidation-reduction reaction between Cr (VI) and ethylene as depicted in eq 5.1. This generates Cr (II) and vacant coordination sites. As mentioned above, polymerization may be initially slow because of sluggish reduction or desorption of the oxidation by-products which can coordinate with (and block) active centers. 

The spin coated samples were activated in a quartz reactor, under a 30 ml/min flow of dry air (99.98 %, Indugas) purified over molecular sieves (3 A, Alltech). The temperature was increased by steps of 100°C, at a rate of 10°C/min. Thermal equilibration of the reactor was ensured by holding each plateau for 1 h. The final activation step was at 650°C during 6 h, reflecting the industrial activation process. The activated wafer was then used in a polymerization reaction. This was carried out in 2 bar ethylene (30 ml/min), purified over molecular sieves, at 160°C during approximatively 30 min. These reaction conditions were chosen to reduce the induction period prior to polymerization. The reaction was then stopped by switching to purified He flow (30 ml/min) at 160°C for 2 h, before cooling the reactor to room temperature. 

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